Why did the northern farmers favor protective tariffs

To reduce competition from foreign grain producers.

Explanation:

Protectionism is a doctrine, a theory that preaches a set of measures to be taken in favor of domestic economic activities, reducing and hindering as much as possible the import of products and foreign competition. Such a theory is used by virtually every country, to a greater or lesser extent. Based on this, northern farmers favored protectionist tariffs to reduce competition from foreign grain producers.

To reduce competition from foreign grain producers.